Question

velocity is a derived unit not a fundamental unit why​

Answer 1

There are several base units used in the International System of Units, and for good or bad, velocity is not one of them. 

Those units whose value depend on other units are derived units. For example, To measure the velocity of a car we must know the distance traveled by car and time taken by the car to travel the distance.

Answer 2

Answer:

There are several base units used in the International System of Units, and for good or bad, velocity is not one of them. The general consensus is that it’s more “natural” to define distance (meter) and time (second) and as base units, and derive velocity a the ratio between them.

However, this was ultimately an arbitrary choice. You could easily construct an equally valid system of units, by adding velocity as a base unit, and removing time or distance. And indeed something very similar to this is routinely done by physicist, who simplify their equations by using Planck units (proposed in 1899 by German physicist Max Planck, as being “natural units”). Specifically, they define the following universal constants as equal to 1: